1. Field of the Invention
This invention is in the field of chromium electroplating. More particularly, the present invention is in the field of plating functional chromium and chromium alloys from a solution containing trivalent chromium where the solution contains minimal ammonium ion.
2. Description of the Prior Art
Chromium is traditionally electroplated from electrolytes containing hexavalent chromium, but many attempts over the last fifty years have been made to develop a commercially acceptable process for electroplating chromium using electrolytes containing only trivalent chromium ions. The incentive to use electrolytes containing trivalent chromium salts arises because hexavalent chromium presents serious health and environmental hazards; hexavalent chromium ion and solutions from which it can be plated have technical limitations including the ever-increasing cost of disposing of plating baths and rinse water. Further, the operation of plating from baths containing substantially hexavalent chromium ion has operational limits which increase the probability of producing commercially unacceptable deposits.
The problems associated with electroplating chromium from solutions containing trivalent chromium ions are primarily concerned with reactions at both the anode and cathode, plating rate, hardness, and thickness of the ultimate coating. Other factors which are important for commercial processes are the material, equipment and operational costs.
In order to achieve a commercial process, the precipitation of chromium hydroxy species at the cathode surface must be minimized to the extend that there is a sufficient supply of dissolved, i.e., solution-free, chromium (III) complexes at the plating surface, and that the reduction of chromium ions is promoted.
U.S. Pat. No. 4,062,737 describes a trivalent chromium electroplating process in which the electrolyte comprises aquo chromium (III) thiocyanate complexes. The thiocyanate ligand stabilizes the chromium ions, inhibiting the formation of precipitated chromium (III) salts at the cathode surface during plating, and also promotes the reduction of chromium (III) ions. United Kingdom patent specification No. 1,591,051 describes an electrolyte comprising chromium thiocyanate complexes in which the source of chromium is a cheap and readily available chromium (III) salt such as chromium sulfate.
Improvements in performance, i.e., efficiency or plating rate, plating range and temperature range, have been achieved by the addition of a complexant which provides one of the ligands for the chromium thiocyanate complex. These complexants, described in U.S. Pat. No. 4,161,431, comprise amino acids such as glycine and aspartic acid, formates, acetates or hypophosphites. As described in that publication, the improvement in performance depends on the ligand used. The complexant ligand is effective at the cathode surface, to inhibit further the formation of precipitated chromium (III) species. It is noted in that patent that the improvement in performance permitted a substantial reduction in the concentration of chromium ions in the electrolyte, without the process ceasing to be commercially viable.
In U.S. Pat. No. 4,278,512, practical electrolytes comprising chromium thiocyanate complexes are described; these electrolytes contain less than 30 millimoles of chromium, the thiocyanate and complexant being reduced in proportion. The reduction in chromium concentration has several desirable effects. Initially, the treatment of rinse waters is simplified; secondly, the color of the chromium deposit is reportedly significantly lighter.
Oxidation of chromium and other constituents of the electrolyte at the anode are known to inhibit plating progressively and rapidly. Additionally, some electrolytes result in anodic evolution of toxic gases. An electroplating bath having an anolyte separated from a catholyte by a perfluorinated cation-exchange membrane, described in United Kingdom patent specification No. 1,602,404, successfully overcomes these problems. Alternatively, an additive, which undergoes oxidation at the anode in preference to chromium or other constituents, can be made to the electrolyte. While a suitable additive is described in U.S. Pat. No. 4,256,548, one clear disadvantage of using a consumable additive is the ongoing expense.
In U.S. Pat. No. 4,612,091, Benaben et al. show the use of trivalent chromium ion in a solution with low pH where the thickness of the plate approaches functional values. The chromium ion is obtained by reduction of chromium trioxide.